There are various types of heat exchangers. Among these, plate-type heat exchangers (flat-plate heat exchangers), which have high heat exchanging performance and can easily be installed and maintained, are widely used for electric water heaters, industrial equipment, car air-conditioners, and the like. A plate-type heat exchanger has a plurality of layered thin metal plates configured so that paths for high- and low-temperature media are formed alternately and adjacently between the layered plates to allow for heat exchange between the high- and low-temperature media through each plate.
As methods of assembling a plurality of plates in a layered manner, Patent Document 1 describes, for example, bonding methods such as tightening with a gasket and a screw, welding, and brazing. For small-to-medium sized heat exchangers, bonding operations are often performed by brazing in view of pressure resistance. However, attempts for layering and bonding corrugated plates by brazing may result in bonding defects inherent to braze materials, such as erosion occurring upon bonding, cracks in brazed portions, and buried flow paths with a braze melt.
Accordingly, use of the diffusion bonding method has been considered for substituting for the brazing method. Diffusion bonding is a bonding method which takes advantage of interdiffusion of atoms of a base material occurring at a bonding interface at high temperature and high pressure under vacuum or an inert atmosphere. A diffusion bonded region can be obtained having such strength and corrosion resistance comparable to those of the base material.
Diffusion bonding is performed by layering and pressing bonding members with a pressurizing means, and maintaining the pressurized state for a predetermined period of time. A release member such as a cover plate and a spacer may be sandwiched between a bonding member and a pressurizing means when diffusion bonding is performed. Such a release member may be made of a carbon material because thermal resistance and anti-breakage properties are required at a temperature during diffusion bonding. For example, Patent Document 2 describes a means for diffusion bonding in which an elastic carbon sheet is used.    Patent Document 1: Japanese Unexamined Patent Application, Publication No. 2010-85094    Patent Document 2: Japanese Unexamined Patent Application, Publication No. 2014-128815